Other security researchers have noted a similar decline, but warn that while the botnet is currently inactive it may yet return, possibly in a more potent form.

Storm front

The malware used to create zombie clients within the Storm botnet first began circulating in January 2007 in emails referencing the lethal storms ravaging Europe at the time. Prospective marks were invited to click links that directed them towards booby-trapped websites that infected Windows PCs.

Over time the gang behind the Storm worm - actually more accurately described as a Trojan - developed a variety of different social engineering lures, including fake electronic greeting cards. No new campaigns have been launched for a month.

At the peak of its activity in September 2007, compromised proxies in the Storm worm botnet were throwing out 20 per cent of the world's junk emails, according to Marshal. Industry estimates at the time (since revised) suggested anywhere between one and ten million Windows PCs had become zombie drones within the Storm worm botnet at the peak of its activity. More detailed studies have since put the figure of compromised clients in the Storm botnet somewhere between 500,000 and one million.

The success of the unknown bot-herders behind the worm inspired similar attacks by other groups, likewise designed to establish a network of compromised machines. These zombie clients were used to distribute junk mail or launch denial of service attacks, either by their owners themselves or (more commonly) by spammers and other miscreants who rented access to compromised machines via underground forums.

"Storm was one of the first botnets to use these tactics on a mass scale," explained Phil Hay, a lead threat analyst at Marshal. "It became the most successful botnet of its type and established the basic template for developing a spam empire that other botnets have since copied."

"Whoever was behind Storm really set the benchmark at the time for the kind of scale that was achievable with a spambot. They also led the way in using self-perpetuating malicious spam to grow the botnet. They utilised every social engineering trick in the book and invented quite a few of their own."

Storm’s success ultimately led to its undoing, especially after Microsoft targeted Storm through the Malicious Software Removal Tool in September 2007. Redmond reported that 274,372 Windows PCs were cleaned up using its tool during the first month alone.

By January 2008 the Storm worm was pumping out two per cent of the world's junk mail as Microsoft's clean-up efforts, as well as competition from rival botnets, ate into the malware's "market share".

Canadian pharmacy spam, pump and dump stock manipulation, and phishing scams were the most common types of nuisance messages distributed by the botnet.

Penis pills and phishing scams

Rival botnets such as Srizbi, Mega-D and Rustock supplanted Storm as the main producers of junk mail. Marshal's figures show a steady decline of spam from Storm botnet clients since the start of the year. The firm reckons Storm-compromised clients may have been subsumed into rival botnets and are continuing to churn out junk mail. However, it justifies its assertion the worm has petered out by noting that the command and control servers directing it are lifeless.

Still, few seriously believe the Storm worm's creators have abandoned the cybercrime business altogether.

"We have seen occasional surviving Storm bot peers still trying to communicate with each other," Hay said, "but the Storm’s command and control servers are unresponsive. Our data indicates that Storm has stopped. Maybe not forever, but the most likely scenario we can envision is that Storm has become obsolete in the face of other botnets like Srizbi which are more resistant against detection and removal by anti-malware solutions.

"A distinct possibility is that the creators of Storm have abandoned it in favour of a newer botnet that they have created. If they have, it is possibly one of the top spam botnets that we continue to track. It seems unlikely that Storm’s creators simply gave up and went home."

Marshal has posted a chart tracing the rise and fall of the Storm botnet here.

Diary of the Dead

Net security blog sudosecure.net backs up Marshal's analysis, with a few qualifiers. "I have not seen any spam, DDOS attacks, or Fast Flux domain activity related to the Storm Worm since mid September," writes sudosecure.net blogger Jeremy.

Jeremy went to the trouble of deliberately infecting a machine with a Storm worm client. He discovered other clients on the Storm p2p network but servers on the TCP control network responded only with the reply: "Go away, we're not home" - evidence that for all their faults, cybercrooks are not without a sense of humour.

It could be that the Storm worm authors have put everything on hold, or are simply lying low so that law enforcement attention drifts elsewhere, Jeremy speculates. "I think the question isn't 'is Storm dead', but more like when will we see it return and what new features or tactics will it have in store for us." ®